Respiratory bronchioles (RB) form the transitional zone between the conducting airways and alveoli in the primate lung. These anatomical units are defined as airways with occasional alveolar outpocketings along their walls. The interdigitation of bronchiolar epithelial cell types with alveolar epithelial cells in the RB represents a unique microenvironment for cell and tissue responses to particles and oxidant pollutants. The purpose of this study was to determine the deposition patterns of particles in RBS of rhesus monkeys. Animals were exposed for a 3 hour period to aerosolized fluorescent yellow-green microspheres 1m in diameter. Within two hours of the end of exposure the lungs were fixed by intravascular perfusion of glutaraldehyde-paraformaldehyde solution. Immediately following exposure, numerous single microspheres were scattered in a patchy distribution along the conducting airways, RBS, and alveolar ducts with the highest concentrations found on the bifurcation ridges. In RBS, more microspheres were found on the alveolated side of the RB wall compared with the non-alveolated side. Microspheres within the RB were scattered on the surfaces of tissue crests forming the mouth openings of the alveoli and into the alveoli. By 24 hours, most microspheres had cleared from the conducting airways. In striking contract, many microspheres remained in RBS and alveoli. The distribution of microspheres at 24 hours in RBS had shifted primarily to small clusters rather than single microspheres. Microspheres on the alveolated side of the RB continued to be the dominant location of spheres in the RB with little clearance from this level in the lungs. We conclude that particles depositing in respiratory bronchioles are preferentially localized to the alveolated portions of the wall, undergo active redistribution following deposition, but unlike particles in the conducting airways, are retained at sites of deposition in RBS for at least 24 hours following inhalation.